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1 of 39© Boardworks Ltd 2007 Kinetics. © Boardworks Ltd 20072 of 39.

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Presentation on theme: "1 of 39© Boardworks Ltd 2007 Kinetics. © Boardworks Ltd 20072 of 39."— Presentation transcript:

1 1 of 39© Boardworks Ltd 2007 Kinetics

2 © Boardworks Ltd of 39

3 © Boardworks Ltd of 39 What does rate of reaction mean? The speed of different chemical reactions varies hugely. Some reactions are very fast and others are very slow. What is the rate of these reactions? The speed of a reaction is called the rate of the reaction. rustingbakingexplosion slowfastvery fast Airbag Reaction

4 © Boardworks Ltd of 39 Rates of reaction Why are some reactions faster than others?

5 © Boardworks Ltd of 39 Measuring the rate of a reaction The change in concentration can be measured by using any property that changes during the reaction. These may include Mass or volume changes for gaseous reactions. Change in pH for reactions involving acids and bases. Changes in conductivity measurements for reactions involving electrolytes. Use of a spectrometer or colorimeter for reactions involving change in colour.

6 © Boardworks Ltd of 39 Collision Theory For a reaction to take place between two reactant particles, three conditions are necessary: The two particles must collide with each other. They must collide in the correct orientation, so that the reactive parts of each particle will come into contact with each other. The reactants must collide with sufficient kinetic energy to bring about the raction.

7 © Boardworks Ltd of 39 Graphing rates of reaction

8 © Boardworks Ltd of 39 Reactant–product mix

9 © Boardworks Ltd of 39 Activation Energy The minimum amount of energy required to bring out the reaction is called the Activation Energy The activation energy (E a ) for the forward reaction is shown by (a) The activation energy (E a ) for the reverse reaction is shown by (b)

10 © Boardworks Ltd of 39 Setting up rate experiments What equipment is needed to investigate the rate of hydrogen production? gas syringe rubber bung rubber connecterglass tube conical flask magnesium hydrochloric acid

11 © Boardworks Ltd of 39 hydrogen produced (cm 3 ) time (seconds) x y Calculating rate of reaction from graphs rate of reaction = x y 20 s 45 cm 3 rate of reaction =2.25 cm 3 /s The gradient of the graph is equal to the initial rate of reaction at that time How can the rate of reaction be calculated from a graph?

12 © Boardworks Ltd of 39 Slower and slower! Reactions do not proceed at a steady rate. They start off at a certain speed, then get slower and slower until they stop. As the reaction progresses, the concentration of reactants decreases. This reduces the frequency of collisions between particles and so the reaction slows down. percentage completion of reaction 100%0%25%50%75% reactants product

13 © Boardworks Ltd of 39 The reactant/product mix

14 © Boardworks Ltd of 39 Changing the rate of reactions increased temperature increased concentration of dissolved reactants, and increased pressure of gaseous reactants increased surface area of solid reactants.(decrease particle size) use of a suitable catalyst. Anything that increases the number of successful collisions between reactant particles will speed up a reaction. What factors affect the rate of reactions?

15 © Boardworks Ltd of 39 Collisions and reactions: summary

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17 © Boardworks Ltd of 39 Temperature and collisions How does temperature affect the rate of particle collision?

18 © Boardworks Ltd of 39 Effect of temperature on rate The higher the temperature, the faster the rate of a reaction. In many reactions, a rise in temperature of 10 °C causes the rate of reaction to approximately double. Why increasing the temperature will increase the rate of reaction?

19 © Boardworks Ltd of 39 Temperature and particle collisions

20 © Boardworks Ltd of 39 Effect of temperature on rate Increasing the temperature will make the particles move faster, so there will be more collisions. At a higher temperature, many more particles will have the necessary activation energy. The ratio of successful collision to unsuccessful collisions will increase. Which one will contribute more towards increasing the rate of reaction?

21 © Boardworks Ltd of 39 How does temperature affect rate? The reaction between sodium thiosulfate and hydrochloric acid produces sulfur. Sulfur is solid and so it turns the solution cloudy. How can this fact be used to measure the effect of temperature on rate of reaction? hydrochloric acid sodium chloride sulfur sodium thiosulfate + + water sulfur dioxide ++ Na 2 S 2 O 3 (aq) 2HCl (aq) 2NaCl (aq) S (s) ++ SO 2 (g) H 2 O (l) ++

22 © Boardworks Ltd of 39 The effect of temperature on rate

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24 © Boardworks Ltd of 39 Effect of concentration on rate of reaction The higher the concentration of a dissolved reactant, the faster the rate of a reaction. Why does increased concentration increase the rate of reaction? At a higher concentration, there are more particles in the same amount of space. This means that the particles are more likely to collide and therefore more likely to react. higher concentration lower concentration The ratio of successful collisions to unsuccessful collisions will stay the same, but there will be more successful collisions.

25 © Boardworks Ltd of 39 Concentration and particle collisions

26 © Boardworks Ltd of 39 The effect of concentration on rate

27 © Boardworks Ltd of 39 Effect of pressure on rate of reaction The gas particles become closer together, increasing the frequency of collisions. This means that the particles are more likely to react. Why does increasing the pressure of gaseous reactants increase the rate of reaction? As the pressure increases, the space in which the gas particles are moving becomes smaller. lower pressure higher pressure

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29 © Boardworks Ltd of 39 Effect of surface area on rate of reaction Any reaction involving a solid can only take place at the surface of the solid. If the solid is split into several pieces, the surface area increases. What effect will this have on rate of reaction? The smaller the pieces, the larger the surface area. This means more collisions and a greater chance of reaction. This means that there is an increased area for the reactant particles to collide with. low surface areahigh surface area

30 © Boardworks Ltd of 39 Surface area and particle collisions

31 © Boardworks Ltd of 39 Reaction between a carbonate and acid Marble chips are made of calcium carbonate. They react with hydrochloric acid to produce carbon dioxide. The effect of increasing surface area on the rate of reaction can be measured by comparing how quickly the mass of the reactants decreases using marble chips of different sizes. hydrochloric acid calcium chloride calcium carbonate ++ water + carbon dioxide CaCO 3 (aq) 2HCl (aq) CaCl 2 (aq) ++ H 2 O (aq) + CO 2 (g)

32 © Boardworks Ltd of 39 The effect of surface area on rate

33 © Boardworks Ltd of 39 reaction (time) energy (kJ) What are catalysts? Catalysts are substances that change the rate of a reaction without being used up in the reaction. Catalysts never produce more product – they just produce the same amount more quickly. Different catalysts work in different ways, but most provide an alternative path with lower activation energy (E a ). E a with catalyst E a without catalyst

34 © Boardworks Ltd of 39 How do catalysts work? More of the reactants will possess this lower activation energy.

35 © Boardworks Ltd of 39 Everyday catalysts Nickel is a catalyst in the production of margarine (hydrogenation of vegetable oils). Many catalysts are transition metals or their compounds. For example: Platinum is a catalyst in the catalytic converters of car exhausts. It catalyzes the conversion of carbon monoxide and nitrogen oxide into the less polluting carbon dioxide and nitrogen. Iron is a catalyst in the production of ammonia from nitrogen and hydrogen (the Haber process).

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37 © Boardworks Ltd of 39 Glossary activation energy – The amount of energy needed to start a reaction. catalyst – A substance that increases the rate of a chemical reaction without being used up. concentration – The number of molecules of a substance in a given volume. enzyme – A biological catalyst. rate of reaction – The change in the concentration over a certain period of time.

38 © Boardworks Ltd of 39 Anagrams

39 © Boardworks Ltd of 39 Rates of reaction: summary

40 © Boardworks Ltd of 39 Multiple-choice quiz


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